Photolithographically processible resists have widely been used in application of semiconductors, MEMS, micromachines in recent years. In such application, a photolithographic process can be attained by pattern exposing a surface of a substrate, and developing the exposed surface with a developing solution to selectively remove an exposed region or a non-exposed region. The photolithographically processible resists (photoresists) are classified into a positive type and a negative type. The positive type is a photoresist in which the exposed region is dissolved in the developing solution, and the negative type is a photoresist in which the exposed region is not dissolved in the developing solution. In high-tech electro package application and MEMS application, not only an ability to form a uniform spin coating film, but also a high aspect ratio, a straight shape of a side wall of a thick film, high adhesion to a substrate, and the like are demanded. Here, the aspect ratio is an important property calculated from film thickness of the resist/width of a pattern line and indicating performance of photolithography.
As such a photoresist, a negative type chemical amplification photoresist composition composed of polyfunctional bisphenol A novolak type epoxy resins (trade name EPON SU-8 Resin, made by Resolution Performance Products LLC) and a photocationic polymerization initiator such as CYRACURE UVI-6974 made by Dow Chemical Company (the photocationic polymerization initiator is composed of a propylene carbonate solution of aromatic sulfonium hexafluoroantimonate) is known. The photoresist composition has extremely low light absorption at a wavelength band of 350 to 450 nm, and is known as a photoresist composition that can be formed into thick-film photolithography. The photoresist composition is applied onto various substrates by a method such as spin coating or curtain coating, and then, the solvent in the composition is volatilized by baking to form a solid photoresist layer having a thickness of 100 μm or more. Further, the photoresist layer is irradiated with near-ultraviolet light through a photomask by a variety of light exposing methods such as contact exposure, proximity exposure, or projection exposure. Thus, the photolithographic process is performed. Subsequently, the substrate is immersed in a developing solution to dissolve a non-exposed region. Thereby, a negative image of the high resolution photomask can be formed on the substrate.
Meanwhile, in the fields of the MEMS parts, MEMS packages, semiconductor packages, and the like, it is known that physical properties of the package material give influences to reliability of the device. MEMS elements and semiconductor elements easily have their properties degraded by change in ambient temperature and humidity or an influence of fine wastes and dust, and are easily broken by mechanical vibration or impact. In order to protect the MEMS elements and the semiconductor elements against these external factors, these elements are used in a form in which the element is sealed by a variety of materials, or a form in which the element is contained inside a hollow structure (cavity) surrounded with an outer wall made of a variety of materials, that is, a form of a package. In the case of hermetic sealing in which metal or ceramics are used as the material for the sealant or the outer wall, the package to be obtained has high reliability, but also has demerits such as high manufacturing cost and poor dimensional precision. Contrary to this, in the case of resin sealing in which a resin is used as the material for the sealant or the outer wall, the conventional resin has relatively low manufacturing cost and high dimensional precision, but has problems with moisture resistance, heat resistance, and the like. Examples thereof are: the sealant is peeled off from the substrate or the element due to moisture absorbed from an external environment by the resin material, or defects are produced attributed to outgas generated from the package when the package is exposed to a high temperature environment. Moreover, in these years, for example, the following problems have occurred: in the package having a cavity provided using a resin material, when the package is quenched from a high temperature heating step such as reflow soldering, moisture contained in the resin or generated by a curing reaction of the resin or the like is condensed inside the cavity to reduce the properties of the MEMS elements and the semiconductor elements.
Patent Literature 1 discloses a photocationic polymerization initiator. According to the description in Examples, using the photocationic polymerization initiator, a photosensitive composition containing 3,4-epoxycyclohexylmethylcarboxylate and 3-ethyl-3-hydroxymethyl-oxetane is cured. In the description, however, it is not found that the photosensitive resin composition containing the photocationic polymerization initiator having a specific structure and the epoxy resin having two or more epoxy groups in one molecule has high adhesion, patterning properties, humidity and heat resistance, and the like. According to the description in Patent Literature 2, in production of a package having a cavity, if a photosensitive resin composition containing a photocationic polymerization initiator having a specific structure and an epoxy resin having two or more epoxy groups in one molecule is used, sensitivity is high and adhesion to the substrate after a pressure cooker test (PCT) is not reduced. However, the effect of reducing condensation inside the cavity is not mentioned at all.